Refactor start.py and test_integration.py

[michaelschwier]

Currently these are very long script files. Refactoring steps:

1. Try a better separation of functionality makes sense, possibly splitting up into classes. However, still keep everything in one file each, such that the user does not have to download several files for starting/testing.

2. Turn the whole thing into an installable python package. Then we can split functionality over several files to make the architecture even nicer. Usage of the package would then be like:
   modelhub start
   modelhub list
   modelhub test_integration